The present invention relates to an improvement of an optical fiber from which a light beam leaks through its peripheral surface. In particular, the present invention relates to an optical fiber comprising a core which propagates light in its lengthwise direction, and two types of light-extracting elements (the first light-extracting reflective element and the second light-extracting light-transmissive element) which are provided along the lengthwise direction of the core on the peripheral surface of the core.
Optical fibers are known, which comprise a core for propagating light which enters from one lengthwise end towards the other end, and reflective means for extracting the light which is provided along the lengthwise direction on the peripheral surface of the core.
The light-extracting reflective means diffusively reflects a part of light beams, which may otherwise reach the other end by repeated total reflection on the inner surface of the core, among the light beams propagating the core, and allows such a part of light beams leak from a part of the peripheral surface opposite to the part of the peripheral surface on which the light-extracting means is provided. Thus, the light beams can be extracted along the lengthwise direction of the core. Such peripheral surface-light-extracting type optical fibers are used as substitutes for glass tube type linear light sources such as fluorescent lamps, cold cathode tubes, neon tubes, and the like.
For example, JP-B-4-70604, JP-B-1-58482, and JP-A-621940 disclose, as reflective light-extracting means, a diffusively reflective membrane comprising a flat resin film containing diffusively reflective particles dispersed therein, the flat surface of which is in contact with a core. Such a diffusively reflective membrane is usually formed from a coated layer of a dispersion containing a light-transmitting resin and diffusively reflective particles, and does not substantially allow the light transmission.
Among the light beams which propagate the core, a part of the light beams, which reach the peripheral surface of the core in contact with the diffusively reflective membrane and are reflected by the diffusively reflective membrane, and then leaks outside the core through the light-extracting peripheral surface. On the other hand, the light beams do not substantially leak outside the core through the diffusively reflective membrane. Thus, such a diffusively reflective membrane is one of the most suitable light-extracting means for increasing light-emission luminance on the light-extracting peripheral surface of the optical fiber.
The diffusively reflective particles are usually inorganic white powders having high refractive indexes. The light-transmitting resin is usually one having a refractive index which is higher than that of the optical fiber material but is different from that of the diffusively reflective particles.
For the increase of the light-emission luminance, in general, the refractive index of the inorganic particles is increased, and the content of the inorganic powder is increased as much as possible.
As explained above, the use of the light-extracting means such as the diffusively reflective membranes allows the light beams to be extracted along the lengthwise direction of the core, and the optical fiber can be used as a substitute for a linear light source such as a fluorescent lamp. However, it has been very difficult to increase the light-emission luminance only with the diffusively reflective membrane, because the refractive index of the organic powder is at most about 3, and it is very difficult to obtain organic powder having a refractive index of higher than 3, or to disperse the large amount of the organic powder in the resin.
The present invention is to provide a peripheral surface-light-extracting type optical fiber having a high light-emission luminance which cannot be attained by the use of only a reflective light-extracting means.